The present invention is concerned with a protective coating with current applied on a metallic collector of an electrode. The protective coating comprises a mineral binder partially or completely vitreous and optionally an electronic conduction additive. This coating is applied in the form of a solution or dispersion on the collector of the electrode and dried, so that it coats and protects at least part of the surface of the metal of the collector to prevent the formation of insulating films generated by active species originating from other components of the generator.
In principle, lithium batteries offer the unique advantage of reaching unsurpassed volumic and specific energies that render them particularly interesting for a wide range of applications, whether stationary or mobile, from microelectronics, portable electronics to large installations for electrical vehicles or hybrids.
These systems generally use liquid aprotic electrolytes mainly of the lithium-ion type, and more recently polymer electrolytes, the latter being either of the dry solvating type polymer, working between 40 and 100xc2x0 C. with lithium anodes, or of the gelified type, using a solvating or non-solvating polymer, and working at room temperature because of the addition of aprotic polar liquid solvents associated with lithium-ion type electrodes having cathodes working at elevated tensions (xcx9c4V). The extreme operating conditions of temperature or tension of these systems cause the various components of the generator to age under cycling and/or in function of the time.
The components aging is evident at the current collectors level, and results in the formation of passivation films or in the degradation of the surface of the collectors because of the reactions between the components of the generator, namely the active materials of the electrodes, and the chemical components of the organic electrolyte. The formation of such films, more or less insulating, at the interfaces, significantly alters the quality of the electronic exchanges between the collectors and the electrode active materials, which are generally present in the form of composite.
In a polymer electrolyte medium, the effect of the passivation phenomenons is sometimes amplified because in the solid state, the products formed by the reactions of the organic solvent, the lithium salt, the electrode materials or from other components of the generator, tend to accumulate at the interface because of the lack of the convection of the solvent or the solubilization of the films formed, or because of the lack of corrosion/dissolution reaction of the metal and the renewal of the exchange surface. The attack of the collectors or the formation of passivation films at the surface by the oxidation-dissolution of the metallic conductor is generally caused by electrochemical reactions, namely oxidation or reduction, initiated by radicals, acid-base reactions or oxidation-reduction chemical reactions more or less catalyzed by the materials present.
FIGS. 1a) and 1b) illustrate a collector/electrode composite assembly and the localization of the passivation film at the interface collector/electrode after cycling.
Passivation phenomenons are particularly evident in the case of aluminium collectors, which are frequently used because of their low cost and their thermal and electrical conduction properties with cathode associated with end of charge voltages frequently higher than 3 and even 4 volts.
In lithium-ion type systems using liquid electrolytes or gelified polymers with liquids, the corrosion of the aluminium of the cathode collector is generally prevented through the use of a salt or a fluorinated additive of the type LiBF4 and LiPF6 that easily form a fluorinated film at the surface of the aluminium, or with an oxidative anion ClO4xe2x88x92, thus preventing deep corrosion or dissolution of the aluminium collector. With other particularly stable fluorinated salts such as TFSI of formula (CF3SO2)2NLi, the corrosion of aluminium above 4 volts can lead to the complete disintegration of the collector.
In dry polymer medium, the formation of passivation films on the aluminium collector of a vanadium oxide-based cathode (V2O5) does not lead to the dissolution of the collector, but rather to the formation of passivation films more or less insulating, that increase the electrical resistance between the collector and the composite cathode. There is then observed the formation of oxygen and fluorine-based oxidation films of aluminium, which are visible under electronic microscopy, that reach thicknesses higher than that of alumina films initially present at the surface of the aluminium. Such films are more or less electrical insulators and thus impair the passage of electrons between the collector and the electronic conduction and active materials present in the cathode.
It has been known for a long time to protect the metallic current collectors of electrochemical accumulators from passivation/dissolution phenomenon by coating the latter with an electronically conductive carbonated coating that is not very oxidizable. Generally, carbon black dispersions in organic or mineral binders are used in the form of a layer more or less impermeable to the electrolyte of the generator to prevent electrochemical corrosion phenomenons. Further, these coatings prevent a direct contact of the collector with the electrode active materials (see for example U.S. Pat. No. 5,262,254). Such solutions are used successfully in various commercial applications. However, none of them is perfectly satisfactory, particularly when the electrochemical generators are used in extreme conditions as described above, and over extended period of times, notably because of the lack of impermeability and chemical or electrochemical stability of the organic binders, or the metallic conduction additives or conjugated polymers.
U.S. Pat. No. 5,580,686 (Fauteux et al.) describes a carbon-based protective coating (xe2x80x9cprimerxe2x80x9d) dispersed in a metallic polysilicate used in an electrolytic cell of the lithium-ion type containing a cobalt oxide cathode and a graphite anode. The polysilicates comprise several limitations because of their strong basicity. For example, they are reactive towards acidic electrode active materials such as vanadium oxide. Further, they are chemically reactive with iron phosphate-type materials. The basic character renders them incompatible with conduction additives made of conjugated polymers of the polyaniline type, doped polypyrole type etc.
In most applications, carbon is generally the preferred additive because of its high chemical inertia and its resistance to electrochemical corrosion.
The present invention is concerned with an electrochemical generator comprising a separating electrolyte between two electrodes, wherein at least one of the electrodes comprises a metallic current collector coated at least partially with a conductive coating, chemically compatible with the adjacent electrode material, the coating comprising a vitreous or partly vitreous mineral binder comprising an alkaline metal phosphate, an alkaline metal polyphosphate, an alkaline metal borate, an alkaline metal polyborate, or mixtures thereof, wherein at least one electronic conduction additive ensuring electronic exchanges between the electrode and the collector is optionally dispersed, the coating being contacted with the collector to protect the coated metallic surface from the formation of passivation films generated by reactive species originating from the generator components.
In a second aspect of the present invention, there is provided a process for coating, at least partly, the surface of a metallic collector of an electrode with a passivation protector, the process comprising:
a) preparing an aqueous dispersion solution of a mineral binder being vitreous or partly vitreous, and comprising an alkaline metal phosphate, an alkaline metal polyhphosphate, an alkaline metal borate, an alkaline metal polyborate, or mixtures thereof that is neutralized to ensure compatibility of the binder with the active material of the electrode and wherein at least one electronic conduction additive is dispersed;
b) coating the surface of the metallic collector with the dispersion prepared in step a), and drying to produce a partly or completely coated surface of the metal to prevent formation of passivation films generated by active species from other components of the generator.
In a third aspect of the present invention, there is provided an electrode comprising a metallic current collector coated in whole or in part with a protective conductive coating, chemically compatible with the adjacent electrode material, the coating comprising a vitreous or partly vitreous mineral binder wherein is optionally dispersed an electronic conduction additive ensuring electronic exchanges between the electrode and the collector, the coating being contacted impermeably with the collector to protect the metallic surface coated from the formation of passivation films generated by reactive species from generator components.